Particulate control in flue gases produced from coal combustion is a complex problem because of the orders of magnitude of particle sizes that one must deal with and the wide range in particulate removal efficiency that is available from various control devices. Particle size in gas process streams may range from 0.01 micron, to 100 micron, where the smaller particles are, for all practical purposes, permanently suspended in the gas stream (i.e. they will not settle out by gravitational forces), while the larger particles are the size of a small grain of sand and will settle out of air at a rate of 5 cm/s. The smallest particles (0.01 micron) are only 1.5 orders of magnitude larger than gas molecules and are very much subject to mass transfer by diffusion. On the other hand, for 100 .mu.m particles, inertial effects dominate mass transfer operations. Since the aerodynamic effects of the largest and smallest particles are grossly different, particle control immediately becomes complex. A good method for removing larger particles may be completely ineffective for small particles. For example, cyclone separators are highly efficient at removing 20 to 100 micron particles, but 0.01 micron to 1 micron particles pass through cyclones with essentially no removal.
Adding to the complexity of particulate control is the fact that removal efficiency may range from 90 to 99.999% of the total particulate mass, depending on the control method. It might appear that there is a small difference between 99 and 99.999% removal, but this represents three orders of magnitude difference in the amount of pollutant material that enters the atmosphere.
Current best control technology for large coal-fired boilers can generally remove from 99 to 99.9% of the incoming particulate mass. Electrostatic precipitators or baghouses are the technologies which have most often been employed to meet the current standard. If properly designed, these technologies have been successful in most cases.
Current technology can achieve 99.9% particulate removal efficiency on a total mass basis, but the removal efficiency for the fine particle fraction may be much less. The present level of control must meet current emission standards and should also allow operation with a clear stack for good public relations. In addition, the emission of fine particles is an issue because of potential adverse health effects and visibility impairment in the atmosphere.
Most particulate collectors are designed for a given dust. In other words, given a dust with a known particle size distribution, concentration, resistivity, composition, etc., control devices are constructed to collect the dust at a certain design efficiency. One exception to this is flue gas conditioning applied to electrostatic precipitation which improves collection efficiency by changing resistivity, a property of the dust. Flue gas conditioning, however, is usually an add-on method which is applied when the original design does not meet expectations or when new regulations require reduced emissions. If it is possible to modify the dust making it easier to collect by conventional methods, which results in a 100 fold reduction in particulate emissions and improves the operability of the collection device, then a method would be available which could reduce particulate emissions to very low levels without a severe economic penalty.
The objective of the present invention is to provide a method of achieving fine particulate matter control in a manner which fully complies with the Clean Air Act, and which reduces visibility impairment of the atmosphere, and the risk of adverse health effects without significant economic penalty to the coal burning facility.
Another objective of the present invention is to condition particulate fly ash matter from coal combustion plants such that it allows baghouse particle controllers to be much more effective.
An even further objective of the present invention is to provide a baghouse particle controller with an injection system that supplies the particle conditioning gases, ammonia and sulfur trioxide, in a manner such that the pressure drop across a baghouse is decreased and the particulate matter exiting to the atmosphere is decreased.
An even further objective of the present invention is to achieve improved particle matter control of the stack emissions from coal combustion plants which employ reverse-air baghouse particle filters.
Still other objectives include development of a process which allows the size of the baghouse to be reduced and which also increases bag life.
The method and means of accomplishing each of these objectives will become apparent from the detailed description of the invention which follows hereinafter.